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Within there is a long central mountain with three peaks. Under a high light the region on the W. is seen to be crossed by broad light streaks. OLBERS. A large ring-plain, 41 miles in diameter, near the limb, N.E. of Cavalerius. Though a very distinct formation, it is difficult to see its details except under favourable conditions of libration.
As the moon, except for the slight movement termed its "libration," always turns the same face to us, so that we see in all only about four sevenths of its surface, it has naturally been conjectured that the unseen side, which is probably some miles lower than that turned toward us, might have a different character from that which we behold. There are reasons why this is improbable.
The remainder or at least most of them are extremely delicate objects, which vary in visibility in a way that is clearly independent of libration or solar altitude; and, what is also very suggestive, they are always found closely associated with the light markings, standing either upon the surface of these features or close to their edges.
The libration of the moon formed a very imperfect part of physical astronomy when Lagrange made it depend on a circumstance connected with the figure of our satellite which was not observable from the earth, and thereby connected it completely with the principles of universal gravitation.
The moon really undergoes considerable libration, recalling the libration of Mercury, which was explained in the chapter on that planet, and in consequence we are able to see a little way round into the opposite lunar hemisphere, now on this side and now on the other, but in the diagram this libration has been neglected.
The three satellites would participate in this libratory movement, the extent of oscillation depending in each case on the mass of the satellite and its distance from the primary, but the period of libration is the same for all the satellites, amounting to 2,270 days 18 hours, or rather more than six years.
Ardour, and not lethargy, progress and not decline, are here represented as the characteristics of extreme old age. An enthusiasm of effort and of strenuous endurance, an enthusiasm of rest in knowledge, an enthusiasm of self-abandonment to God and the divine purpose make up the poem. At no time did Browning write verse which soars with a more steadfast and impassioned libration of wing.
Let us compare Taylor's treatment of the same image: "For so have I seen a lark rising from his bed of grass and soaring upwards, singing as he rises, and hopes to get to heaven and climb above the clouds; but the poor bird was beaten back by the loud sighings of an eastern wind, and his motion made irregular and inconstant, descending more at every breath of the tempest than it could recover by the libration and frequent weighing of his wings, till the little creature was forced to sit down and pant, and stay till the storm was over, and then it made a prosperous flight, and did rise and sing as if it had learned music and motion of an angel as he passed sometimes through the air about his ministries here below."
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